well, unless you're planning on switching to Vista, i wouldn't worry about it till the next generation of cards comes out. at least not too much, at any rate. you won't be able to play the latest games at max settings, but at least they'll work. the upside is that the 6800's were top of the line a couple years ago; so they should have enough horsepower to keep pace for a little while yet.
if you really want to upgrade, the performance bargains right now are the Radeon 3850's. if you get it from somewhere like Newegg.com, you can get one for under $200 which is a pretty good deal in my book. if you can spend $300 or more, i'd go with the GeForce 8800GT. excellent card that's actually quite affordable. don't go for the GeForce 8600's or the Radeon 2600's. they're good for multimedia but mediocre for gaming.
and since this is a "tech" thread, i'm moving it to our tech forum.
See the struggle of the faithless lot as they negate their time
How low to sink to the depths of their frame of mind